(a) Field of the Invention
The present invention relates to an inversion developing method for electrophotography as well as apparatuses relevant thereto. To be precise, it relates to a wet inversion developing process and a magnetic brush inversion developing process for obtaining an inversion-developed positive image through negative projection applied to an electrophotographic sensitive material from a microfilm, an optical fiber memory tube or the like, as well as apparatuses ideal for practicing these processes.
(b) Description of the Prior Art
In the field of electrophotography, it has so far been considered very difficult to obtain an image having a mild edge effect and a satisfactory gradation according to the so-called `inversion developing method` for making the toner adhere to the non-charged portions of the surface of the photosensitive material. This difficulty is ascribable to the fact that, in the conventional electrophotographic inversion developing method, the apparatus employed has been one which is intrinsically suitable for positive-positive development, and the negative-positive development has been conducted by merely changing the charged polarity of the toner.
It is admittedly known that, according to the magnetic brush inversion developing process, iron powder constituting the carrier acts as a developing electrode whereby there is obtained an image free of edge effect and showing a satisfactory reproducibility with respect to the solid image area in postive-positive development. However, when negative-positive development is conducted by changing the polarity of toner to the same polarity as that of the electric charge on the photosensitive material by friction with iron powder, there is apt to take place such undesirable phenomena that (1) the carrier, i.e., iron powder, mainly adheres to the non-image area (to wit, the area where the charge remains) of the photosensitive material and (2) the toner scarcely adheres to the image area (to wit, the noncharged area). The reason is that, inasmuch as both the developing sleeve and the base of the photosensitive material are earthed, the coulomb's force of the negative charge on the non-image area of the photosensitive material and the inductive charge of the carrier overcomes the magnetic force of the magnet working on the carrier, whereby the quantity of carrier adhering to the non-image area becomes relatively great, while the quantity of toner moving toward the image area along the electric field between the non-image area of the photosensitive material and the developing sleeve becomes to be relatively little.
In order to obtain a negative-positive image through the wet inversion developing process, it is usual to adopt the technique of increasing the bias voltage at the time of developing (to be more precise, imparting a voltage sufficient to overcome the maximum potential of the photosensitive material to the developing electrode). On this occasion, for the bias voltage, a means of utilizing an external power source such as disclosed in Japanese Patent Publication No. 7104/1958, a means of introducing the primary charge current into the developing electrode through ballista, etc. are usually employed.
In the case of the magnetic brush inversion developing process, a means of imparting the bias voltage to the developing sleeve, that is, a means of negatively charging the developing sleeve by the use of an external power source and minimizing the potential gradient between the developing sleeve and the nonimage area of the photosensitive material, thereby preventing adhesion of the carrier to the non-image area, is usually adopted.
However, these conventional processes employing particular devices such as external power source have a drawback that the developing apparatus per se comes to be complicated. Besides, the magnetic brush inversion developing process has also a drawback that, inasmuch as the carrier which is supposed to be positively charged due to friction with the toner is used upon charging negatively, there is a fear of deterioration of the developer, and the photosensitive material is apt to give rise to partial dielectric breakdown or leak through the carrier, resulting in a failure to obtain a satisfactory negative-positive image. Further, although it is generally believed that the toner contributes to improvement of the reproducibility of the half-tone corresponding to the potential gradient between the carrier and the image area of photosensitive material, the result of actual experiments verifies that it is in fact impossible to obtain a satisfactory inversion image by this process.